1. Field of the Invention
The present invention relates to a compensation table generation method, apparatus and computer program product for generating a compensation table used when generating an optical tomography image by OCT (Optical Coherence Tomography) measurement. The invention also relates to a tomography image processing apparatus using the same.
2. Description of the Related Art
The use of optical tomography image obtaining systems employing OCT measurement is proposed when obtaining optical tomography images of living tissues. Such systems are applied to the observation of various regions ranging from fundus or anterior chamber of an eye, or skin to the observation of an artery wall using a fiber probe or of a digestive organ using a fiber probe inserted through the forceps channel of an endoscope. In the optical tomography image obtaining system, low coherence light emitted from a light source is split into measuring light and reference light, then reflection light or backscattered light from a measuring object when the measuring light is irradiated on the measuring object is combined with the reference light, and an optical tomography image is obtained based on the intensity of interference light between the reflection light and the reference light. Hereinafter, reflection light and backscattered light from a measuring object are collectively referred to as the “reflection light”.
The OCT measurement is largely grouped into TD-OCT (Time Domain OCT) measurement and FD (Fourier Domain)-OCT measurement. The TD-OCT measurement is a method for obtaining a reflection light intensity distribution corresponding to a position in the depth direction (depth position) of a measuring object by measuring interference light intensity while changing the optical path length of the reference light.
The FD-OCT measurement is a method for obtaining a reflection light intensity distribution corresponding to a depth position of a measuring object by measuring interference light intensity with respect to each spectral component of the light without changing the optical path length of the reference light, and performing frequency analysis, typically a Fourier transform, on the obtained spectral interference intensity signals using a computer. The FD-OCT does not require the mechanical scanning used in TD-OCT, so that it has been drawing wide attention as a method that allows high speed measurement. Typical systems that use FD-OCT measurement are SD-OCT (Spectral Domain OCT) system and SS-OCT (Swept Source OCT) system as described, for example, in U.S. Pat. No. 5,956,355 (Patent Document 1), and a non-patent document “Three-dimensional and high-speed swept-source optical coherence tomography for in vivo investigation of human anterior eye segments” by Yoshiaki Yasuno et al., OPTICS EXPRESS, Vol. 13, No. 26, pp. 10652-10664, 2005 (Non-Patent Document 1).
Here, in the FD-OCT measurement, a detected interference signal represents interference intensity with respect to wavelength, while an interference signal representing interference intensity with respect to wave number is required in the frequency analysis for obtaining tomography information from the interference signal. Consequently, in the Non-Patent Document 1, wavelength sweep characteristic is calculated from an interference signal obtained with a mirror as the sample, and the interference signal obtained as interference intensity with respect to each wavelength is converted to an interference signal representing interference intensity with respect to each wave number based on the calculated wavelength sweep characteristic. In the Patent Document 1, information of wavelength sweep characteristic of the light source with respect to time is obtained in advance. Then, based on the timing when an interference signal is observed and the wavelength sweep characteristic, the interference signal is converted to an interference signal with respect to wave number, which is further converted such that the interference intensities are evenly spaced apart with respect to a wave number k.
Provision of a conversion table by measuring wavelength sweep characteristic of light in advance, as in the Patent Document 1 and Non-Patent Document 1, however, causes a problem that it takes time and effort. Consequently, it has been anticipated that the conversion table be created more easily.
In view of the circumstances described above, it is an object of the present invention to provide a compensation table generation method, apparatus and computer program product capable of generating a compensation table used for data conversion of an interference signal obtained by OCT measurement more easily. It is a further object of the present invention to provide a tomography image processing apparatus using the same.